I've looked for this data on the net before and was surprised I wasn't able to find it. Thanks for posting. Did you find it on a website or did you go somewhere to get it ? Its funny, all the cold spells we got made it seem like this years hours would have been much higher than they are. Now I know why my Santa rosa plum isn't setting much fruit. Or maybe I just trimmed it to hard.

It is easy to collect this data. You have to select the station like the Phoenix Greenway station, select the hourly data link for a specific year and wait for all the data to load, then select it all and copy it.

Once you have a copy of it, past it into cell A1 of an openoffice spreadsheet. Openoffice is better than MS Office and is free.

Once the data is pasted in column A and column A is selected, use the menu item DATA-TEXT TO COLUMNS. Be sure to select the comma delimiter. The data is separated by commas. This will make columns of the original data.

Now select the whole sheet and sort by column D which is the hourly temperature column in degrees Celsius. Then scroll down and count the number of rows with a column D value between 0 and 7 degrees. An easy way to do this is largest row # - lowest row # +1. Ex. to photo copy book pages 20 to 30 requires 11 pages (30-20+1). Beats counting hundreds of rows one at a time.

You are done in a few minutes and have yearly chill hours. This in not a winter of one year to the next (ex. fall 2009 to spring 2010). This data is from Jan 1 to Dec 31. It works fine to find averages. If you want specific winter data, you will have to select only the data after Sept for the previous year and find the chill hours. Then do the same for the next year's spring data and add the two numbers. This is a bit laborious and not needed to get a general average of winter chill hours for a specific area. I would average at least 5 years to get a better average. As you can see from the data, one year varies considerable from the next. Just select fruit that requires less than 450 hours.

I am going to try the following here in north Phoenix:

Flavor Delight Aprium

Patterson Apricot - mid season

Autumn Glo Apricot - late season

Rosbrough Blackberry - recommended by U of A

Splash Pluot

Flavorella Plumcot

August Pride Peach - late season

Zee Sweet Nuggets 4 in 1 Pluot

Royal Lee Low Chill Sweet Cherry

Minnie Royal Low Chill Sweet Cherry.

Spice Zee Nectaplum

If anybody has tried these varieties for several years, let me know how they turned out.

You'll have to let me know how your cherries turn out. I saw at the fruit tree sale they were selling those varieties, and they are supposed to do well here. Have you had them long enough to get any fruit ?

¨ Select varieties that have a chilling requirement at least 20% less than local averages.

¨ Selecting a low chill variety in a cold area will result in trees flowering too early and being damaged by late frosts.

¨ Selecting a high chill variety in warm areas will result in little or no fruit production.

¨ Early flowering varieties are best in warm climates, late flowering varieties are best in cooler areas.

¨ Early ripening varieties are best in areas with intense summers, late ripening varieties are best in cooler summers.

¨ Climate extremes may eliminate certain varieties that would otherwise meet the chilling requirements. For example, the very dry air and intense summer heat as found in Phoenix Arizona may stress a fruit tree beyond its ability to produce quality fruit.

¨ Terrain can affect the chilling hours received. Open slopes may receive more chilling hours than sheltered areas next to warm buildings.

¨ Various sellers of fruit trees publish significantly varying chilling hour requirements for the same variety. It is difficult to know the exact requirements. Experiment and ask around for promising local cultivar success stories.

Following the above guidelines here is a practical example. A good apricot for Phoenix Arizona (350 chilling hours) would be Katy apricot with a 200-300 chilling hours requirement. It is early blooming and ripens in May and the tree itself thrives in the intense dry desert heat with adequate regular irrigation. The Katy apricot has no apparent pests or disease problems locally. Planting a Katy apricot only 100 miles north (1000+ chill hours) would likely be fruitless from late frost damage to the flowers. A late ripening apricot variety like Autumn Glo might be a bad choice for Phoenix because the intense long summer heat (115+) might cook the green fruit on the tree and result in strange tastes and other problems with late ripening. The same late ripening variety might also fail in the colder areas 100 mile north because of the shorter summer not allowing enough time to properly ripen before cold weather sets in. A better apricot choice for that colder area might be Goldcot with an 800 chill hour requirement. The late ripening Autumn Glo might be better off in a long cool summer climate along the west coast.

How to find local chilling hours data

Local chilling data is as hard to come by as it is to find agreement on what specific fruit tree varieties require. You can find local meteorological weather data all across America and do the calculations yourself rather easily. Then once you have the data you should share it with others by posting on the web.

It is easy to collect this data. You select a local station like the Phoenix Greenway station, select the hourly data link for a specific year and wait for all the data to load, then select it all and copy it.

Once you have a copy of it, past it into cell A1 of an Openoffice or Excel spreadsheet. Openoffice is free and a very powerful replacement for Microsoft Office.

Once the data is pasted in column A and column A is selected, use the menu item DATA-TEXT TO COLUMNS. Be sure to select the comma delimiter. The data is separated by commas. This will make several columns out of the original bunched up data.

Now select the whole sheet and sort by column D which in my case is the hourly temperature column in degrees Celsius. Then scroll down and count the number of rows with a column D value between 0 and 7 degrees Celsius (32F to 45F). Each row represents one hour. An easy way to do this is largest row # - lowest row # +1. For example, to photo copy book pages 20 to 30 requires 11 sheet of paper (30-20+1). Sure beats counting hundreds of rows one at a time. Double check you answers. For some reason, the hours below freezing do not count towards chilling hours.

You are done in a few minutes and have gross yearly chill hours. Easy and quick. This is not a winter of one year to the next (ex. fall 2009 to spring 2010). This data is from Jan 1 to Dec 31. It works fine to find averages. If you want specific winter data, you will have to select only the Fall data for the previous year and find the chill hours. Then do the same for the next year's Spring data and add the two numbers. This is a bit laborious and not needed to get a real average of winter chill hours for a specific area. I would average at least 5 years to get a better average. As you can see from the data below, one year varies considerable from the next.

Finally you need to correct for the reduction in chilling hours caused by warm weather. Since it is not easily known you have to guess. A good guess for Phoenix is reducing the average calculated by about 1/3 to be on the safe side. Phoenix commonly has warm spells during the winter. On 1-1-2012 the high in Phoenix was 80 degrees. These warm spells certainly will undo some of the previous chilling hours. In January some of the last leaves are still on the plums, peaches, and nectarines and the flower buds on some of the varieties are already starting to swell. The dormant bare root fruit trees from California have not even arrived yet at the local nurseries. This tells me if we don’t get a real cold spell soon, the effective chilling hours this year is closer to 300 than to 400.

Now post the data on local gardening web sites for other to enjoy your efforts.

Thanks Sam for bumping this chill hour discussion bacl up and for the link, explanation of calculations and discussion.

It should also be pointed out that there are several chill hour models. I prefer the simplest, but some models subtract chill hours for every hour above a given temp....

And best of luck on the 400 chill hour trees...you are a year ahead of mine which will be planted along an east block wall that is the coldest part of the property and maybe the northern part of the yard which also frosts up.

With the trend this is showing and the comment you have about selecting varieties with 20% less than the local average do you think a pink lady apple tree would be a mistake to plant now? What I found online says 200-400 chill hours and since phoenix is 350, I'm wondering what that would mean for me in terms of production...

Pink Lady and other lower chill apples are a lot more forgiving about chill hours than stone fruit like peaches. Apples, and to some extent, pears, are only partly dependent on chill for blooming, which means that they will fruit successfully even if the chill hours are less than optimal.

I put together a chart of the various locations for chill hours and updated it for last year posting it on a link in a thread here - the climate site is easy to use, but can take some time, so hopefully this is a short cut for folks.

The weather site does the calculation for the stated total hours - calculated from November 1 through March 31. They consider that the primary calculation criteria for hours below 45. I had a very nice gentleman from there walk me through understanding the data the first year I began the chart - 2004 I think. If you want to check out the site, go to the link Sam provided find a station, say Mesa. Click on it. Go to 2011 and click on "weekly" and pick the end of the month of March, then look at April for the total for the 2010/2011. Hope that helps in understanding what I set up.